1. Field of the Invention
The present invention relates to an apparatus for and a method of manufacturing an optical fiber preform, and more particularly to an optical fiber preform manufacturing apparatus and method for more effectively shrinking and closing a deposited tube.
2. Description of the Related Art
Manufacture of optical fiber preforms using a modified chemical vapor deposition (MCVD) method is well known. In a deposition process in which a deposited layer is formed in the form of a tube on the inner surface of a preform tube in accordance with the MCVD method, the deposited tube is shrunk by itself, that is, the internal diameter of the deposited tube decreases, as the thickness of the deposited layer increases gradually. To this end, a burner configured to have a large heating area while using a low flame pressure is typically used in the deposition process. The large heating area of the burner makes it possible to allow an easy heat transfer to the inner portion of a preform tube, thereby achieving an improved deposition efficiency. On the other hand, the low flame pressure of the burner results in a reduced undesirable shrinkage of the deposited tube occurring during the deposition process. However, it is necessary to use a high flame pressure in a tube shrinking and closing process to be conducted for the deposited tube, as different from the deposition process. Where a burner suitable for the deposition process is used for the shrinking and closing process, in spite of the above mentioned fact, it is necessary to move the burner at a low speed while keeping a high heating temperature in order to allow the deposited tube to be softened at the low flame pressure used. As a result, the processing time of the tube shrinking and closing process occupies a large portion of the entire processing time of the optical fiber preform manufacture process. For this reason, the conventional tube shrinking and closing process serves as a great obstacle to a reduction in processing time.
The high heating temperature and low moving speed of the burner in the tube shrinking and closing process result in a degradation in the optical characteristics of optical fibers finally produced, as follows. That is, a trace of moisture (generally, several ppm) contained in the preform tube penetrates into the layer deposited on the inner surface thereof. The penetrated moisture is coupled to P2O5 or SiO2 of the deposited layer, thereby forming P—O—H or Si—O—H bonds. OH reaching the core region of the deposited layer is coupled to SiO2 or GeO2 of the deposited layer, thereby forming Si—O—H or Ge—O—H bonds while releasing Si—O or Ge—O bonds. These O—H or P—O—H bonds resulting from the coupling of the moisture to the compounds of the deposited layer at respective regions of the deposited layer serve to generate an optical loss in the optical fiber, finally produced, due to an absorption band established in a particular wavelength range. OH penetrated into the core layer forms mono-oxygen, thereby resulting in a degradation in the structural uniformity of the vitreous components in the core layer. This causes a non-uniformity in density of the core layer. As a result, an increase in scattering loss occurs.
Second, where the preform tube is heated in a rotating state at its lower end by the burner, a circumferential temperature gradient occurs. The temperature gradient results in a non-uniformity in viscosity of the preform tube. As a result, the balance of surface tension in the deposited tube is lost, thereby causing the deposited tube to be deformed in shape. This causes an increase in the non-circularity of the deposited tube. As the shrinking process advances, the non-circularity of the deposited tube increases, thereby resulting in an increase in polarization mode dispersion.
Conventionally, the deposition process, shrinking process, and closing process have been conducted using the same burner, which is suitable only for the deposition process, in spite of the fact that those processes involve different mechanisms, respectively. For this reason, an increase in the thickness of the deposited layer occurs. This causes various problems such as a deformed geometrical structure of the deposited tube, a degradation in the optical characteristics of optical fibers finally produced, and an increased processing time.
Examples of apparatus and methods for making optical fiber preforms of the conventional art are seen in the following U.S. Patents.
U.S. Pat. No. 3,892,916, to Miller, entitled Method For Forming Optical Fiber Preform, describes a modified chemical vapor deposition process in which asymmetric heating is used to produce circumferentially alternating deposits of doped and undoped glass.
U.S. Pat. No. 4,154,591, to French et al., entitled Fabrication Of Optical Fibers With Improved Cross Sectional Circularity, describes a method for collapsing a tubular preform under positive internal pressure.
U.S. Pat. No. 4,636,236, to Glessner et al., entitled Method For Producing A Preform For Drawing Optical Fibers, describes a method for making a preform in which a tubular glass body having a doped layer is collapsed with a partial vacuum formed in the interior of the tubular glass body.
U.S. Pat. No. 4,820,322, to Baumgart et al., entitled Method Of And Apparatus For Overcladding A Glass Rod, describes a method for collapsing a tube onto a preform rod in which the pressure inside the tube is maintained at a value which is less than that outside the tube.
U.S. Pat. No. 5,658,363, to Ince et al., entitled Method Of Joining A Tube To A Rod Having An Annular Rib, So As To Form An Optical Fiber Preform, describes a method of collapsing a tube onto a rod in which the collapsing step comprises applying suction to the annular space at one end of the assembly.
U.S. Pat. No. 5,917,109, to Berkey, entitled Method of Making Optical Fiber Having Depressed Index Core Region, describes a method of making an optical fiber preform in which an overclad tube is collapsed onto a rod.
Based on our reading of the art, then, we have decided that what is needed is a faster method of manufacturing an optical fiber preform which avoids the above-mentioned problems.